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Article

Heat Treatment of Calcite to Enhance Its Removal of Color Dye Alizarin Red S

by
Zhaohui Li
1,*,
Anna Bowman
1,
Angie Rayniak
1,
Jadyn Strommen
1,
Lori Allen
2 and
Shangping Xu
3
1
Department of Geosciences, University of Wisconsin—Parkside, 900 Wood Road, Kenosha, WI 53144, USA
2
Department of Chemistry, University of Wisconsin—Parkside, 900 Wood Road, Kenosha, WI 53144, USA
3
Department of Geosciences, University of Wisconsin Milwaukee, 3209 N. Maryland Ave., Milwaukee, WI 53211, USA
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(5), 450; https://doi.org/10.3390/cryst14050450
Submission received: 14 April 2024 / Revised: 29 April 2024 / Accepted: 6 May 2024 / Published: 8 May 2024
(This article belongs to the Special Issue Porous Materials and Their Adsorption Behaviors)
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Abstract

:
The use of color dyes in modern society presents a great challenge to the environment. Thus, extensive studies have been conducted in the last 30 years on the removal of color dyes from aqueous solutions such industrial wastewater. In this study, the removal of alizarin red S (ARS), an anionic dye, from solution by raw calcite (Cal) and heat-treated calcite (HCal) was conducted and compared under different physico-chemical conditions. Based on the isotherm study, the ARS removal capacities increased from 167 to 251 mmol/kg after the Cal was heated to 1000 °C for 3 h. The X-ray diffraction analyses showed no difference in the calcite phase between Cal and HCal after ARS sorption. Fourier-transform infrared results also showed no change in the calcite phase after ARS sorption, except a slightly increase in wavenumber from 713 to 727 cm−1 for the OCO bending of HCal at high ARS sorption levels. SEM observations showed about the same particle size and morphology before and after ARS sorption. The TGA data showed the formation of CaO after Cal was heated, and CaO converted back into calcite after being in contact with water or ARS solution for 24 h and then being air-dried. Thus, the high ARS removal could be due to CaO produced after Cal being heated. The findings from this research proved that there is great potential in the use of calcite, a low-cost and readily available Earth material, after heat treatment for the removal of contaminants from water.

1. Introduction

In today’s society, about 1.6 million tons of dyes are produced annually and used in industries including textile, paper, plastics, marine, culinary, paints, and coatings [1]. Just in the textile industry alone, after the dyeing processes are finished, the leftover dye discharged into the environment may be as high as 85% of the initial dye [2], which accounts for the major driving force behind studies for dye removal over the last several decades. Mechanistically speaking, the removal of dyes could be achieved through physical, chemical, and biological processes [2]. For the physical removal of color dyes from solution, sorption is a simple yet effective method provided that the sorbents are inexpensive in material cost, have a wide availability, and have a high removal capacity. Earth materials, due to their low material cost and high material reserves, have attracted great attention for the removal of color dyes [3,4].
Dyes are classified as cationic, anionic, nonionic, and zwitterionic depending on their charges in solution. As most Earth materials bear negative charges on their surfaces, they have great potential for the removal of cationic dyes from solution [5], although the removal of zwitterionic and anionic dyes by clay minerals has also been studied [6,7,8].
Alizarin red S (ARS) is an anionic dye. Originally, it was derived from the roots of plants of the madder genus, and structure-wise, it is made of typical dihydroxyanthraquinone [9]. Initially, ARS was used for the identification of carbonate minerals [10]. Later, it was found that ARS stain on certain carbonate minerals such as aragonite, calcite, witherite, and cerussite was more obvious, as these minerals dissolve more rapidly in a dilute hydrochloric acid system. In contrast, dolomite, siderite, magnesite, and rhodochrosite reacted much slower with the acid and could not be stained by ARS [11]. In comparison to carbonated minerals, ARS sorbed more on apatite, and the OH groups rather than the –SO3 group were responsible for ARS sorption [12].
ARS was also studied for the determination of aluminum content in silicates, as the reaction between aluminum and ARS requires two ARS molecules to form complexation with one atom of aluminum [13]. Later, a patent was granted for the determination of Al(III) by ARS [14]. However, at pH 4.55, the presence of Ca2+ could form a complex between Ca2+ and ARS, which would affect the quantification of Al(III) [15]. The formation of ARS with Ca2+ explained why ARS was used for Ca stain [16]. In biology, ARS staining has been used to evaluate calcium-rich deposits in cells. This staining method is particularly versatile, as the dye can be extracted from the stained monolayer and assayed [17]. Potentiometric titration showed that Ca(HARS)22− dominated at pH 5 to 8 and CaARS− was the major species at pH > 8 [18]. Also, on surface of CaF2, the complexation of ARS with Ca2+ could also form [19]. In addition to Ca, ARS may also react with Mg and complex with other divalent transition metals [9].
Due to its anionic nature, materials for ARS removal are mostly organic as well as synthetic carbon nanotubes [20]. An activated carbon with a mesh size of 40–60 and surface area of 300 m2/g was tested for the removal of ARS, and the calculated ARS sorption capacity was ~7 mg/g based on the Langmuir sorption isotherm model [21]. For inorganic materials, calcium hydroxide was tested for the removal of ARS under different physical and chemical conditions, but no ARS sorption capacity was provided [22]. Additionally, talc was coated with magnetite nanoparticles to enhance ARS removal with a capacity of 12 mg/g [23]. ARS removal from aqueous solution was also evaluated under different physical and chemical conditions using alumina, but the removal capacity was not mentioned [20]. A bentonite in sodium form was tested for ARS removal under different physico-chemical conditions with an ARS sorption capacity at 170 mg/g [24]. ARS removal was also tested using layered double hydroxide (LDH) due to its net positive charges on the surfaces [25]. In most of these studies, the mechanism of ARS removal was not discussed.
In addition to the use of raw materials for ARS removal, inorganic materials were also modified to enhance ARS removal. For instance, activated clay was modified by iron oxide to enhance the ARS removal capacity, but no comparison was made with the clay without iron oxide modification, and the X-ray diffraction (XRD) data showed that the dominant mineral was quartz, not clay mineral at all [26]. A composite material made of Fe2O3-coated biochar/kaolin showed ARS removal. Although the biochar and Fe2O3-coated biochar were tested as the controls, the kaolin alone was not evaluated for the comparison [27].
The mechanism of ARS sorption on clinoptilolite, a zeolite group mineral, was attributed to ligand formation between the Ca2+ on clinoptilolite surfaces and the O on the second carbon of ARS or the O with SO3, as the ARS removal capacity was 92 mmol/kg, close to the external cation exchange capacity of the mineral, and results from molecular dynamic simulations suggested aggregations and dimer formations on the mineral surfaces [4].
In addition to the previously mentioned minerals, carbonated minerals such as calcite (Cal) and dolomite (Dol) were also evaluated for their removal of color dyes before and after heat treatment. For example, Dol heated to 800 °C was tested for the removal of single and binary dyes [28], and Dol heated to 300, 700, and 900 °C was tested for the removal of an anionic dye, methyl orange (MO) [29]. Because of the strong affinity of ARS for calcium and phosphorus, CaCO3 and H3PO5 were utilized to synthesize calcium phosphate hydroxyapatite (Ca-Hap), which was tested for its removal of ARS under different physical and chemical conditions [30]. The removal of the anionic dye acid black 210 by Cal reached a capacity of 210 mg/g [31]. Although the removal of yellow 84 azo dye, also an anionic dye, by amorphous Cal reached a capacity of 40 mg/g [32], the XRD data in the report showed crystallinity of Cal instead of amorphous Cal.
No report, however, could be found for the removal of ARS dye by heat-treated calcite. This knowledge gap represents the primary driving force for this study. Briefly, the removal of ARS by heated Cal was quantified under different physical and chemical conditions. The raw Cal was tested under the same conditions to compare the effectiveness of heat treatment. The interactions between ARS and Cal or HCal were interpreted from the results of experiments and instrumental analyses, and the interaction mechanism was proposed.

2. Materials and Methods

2.1. Materials

The calcite (Cal) was obtained from the mineral collections of the Geosciences Department, University of Wisconsin–Parkside. XRD analyses were conducted first to confirm its purity. The sample was crushed and then ground to less than 0.06 mm. This was the raw calcite serving as the control for comparison. For heat treatment, the ground Cal was placed into a ceramic crucible and heated to 1000 °C for 3 h in a muffle furnace. The heat-treated (HCal) samples were allowed to cool down to room temperature (23 °C) in the oven.
The ARS dye has an IUPAC name of sodium, 3,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate, a formula of C14H7NaO7S, a molecular mass of 342.253 g/mol, and a CAS# of 130-22-3. Its pKa values are listed as 5.49 and 10.85 [9] with Na+ as the counterion. Figure 1 shows its speciation under different pH values. Its solubility in water is up to 1 g/L [33]. And its melting point is 287–298 °C [34].

2.2. Batch Sorption Studies for ARS Removal

For all studies, the solid used was either 0.25 or 0.5 g, and the solution used was 10 mL. For the isotherm study, the initial ARS concentrations were up to 2.5 mM for Cal and up to 10 mM for HCal. The equilibrium ionic strength was adjusted to 0.001, 0.01, 0.1, and 1.0 M NaCl for the ionic strength study. For the pH study, concentrated HCl or NaOH were added periodically to adjust the equilibrium pH to 4–12. For the temperature study, 23, 33, 43, and 53 °C were maintained during shaking. The initial ARS concentrations were 0.5 and 6.0 mM for Cal and HCal for the pH and ionic strength studies and 1.0 and 6.0 mM for the temperature and kinetic studies. The samples were mixed for 24 h at 150 rpm on a mechanical shaker. For the kinetic study, samples were shaken for varying amounts of time from ¼ to 24 h. After shaking, all samples were centrifuged at 3500 rpm for 15 min, followed by filtering the supernatants with 0.45 µm syringe filters. The equilibrium ARS concentrations in the supernatants were determined by a UV-Vis method at a wavelength of 420 nm with proper dilution so that the final absorbance values were less than 1.000. The amount of dye sorbed on solid was calculated by the following: Cs (mmol/kg) = (Cinitial − Cequilibrium)*volume of liquids (mL)/mass of solid (g).

2.3. Instrumental Analyses

The wavelength used for the ARS measurement was 420 nm. Calibrations were made using ARS concentrations between 0.00 and 0.10 mM. For the pH and ionic studies, the standards were made at pH 5, 7, 9, and 11 or at ionic strengths of 0.01, 0.1, and 1.0 M NaCl. The solid samples after dye sorption were air-dried naturally and analyzed by XRD and Fourier-transform infrared (FTIR) to determine whether changes in structure or bonding occurred after dye sorption.
A Shimadzu 6100 X-Ray Diffractometer (Columbia, MD, USA) was used for XRD analyses. CuKα radiation at 30 kV and 40 mA with a Ni filter was used. Samples were scanned at 20–60° (2θ) with a speed of 2°/min. A Shimadzu IRXross FTIR spectrometer with a quartz ATR was used for FTIR analyses of the samples after dye sorption. The scan was performed from 400 to 4000 cm−1 with a resolution of 2 cm−1. A Shimadzu TGA-50 Analyzer was used for the thermogravimetric measurements. Samples were weighed into platinum crucibles and heated at 20 °C/min to 900 °C.
The SEM observation was performed using a Hitachi S-4800 scanning electron microscope. Samples were loaded onto 13 mm aluminum stubs and then coated with carbon using a vacuum coater (Edwards 306A Coating System). SEM images were taken using 10.0 kV accelerating voltage at a 15.0 mm working distance. The selected elements of each sample were scanned by energy-dispersive X-ray spectroscopy (EDS) (Bruker Quantax Esprit System).

3. Results

3.1. Isotherms of ARS Removal

Under different initial ARS concentrations, the amounts of ARS removed by Cal and HCal varied significantly. As such, ARS removal was plotted against equilibrium ARS concentrations (Figure 2), and data were fitted to several isotherm models. The Langmuir model is as follows:
C s = K L S m C L 1 + K L C L
where CL and CS are the ARS concentration in solution (mmol/L) and sorbed on the solid (mmol/kg). The sorption capacity is Sm (mmol/kg), while the affinity of the ARS for the solid is KL (L/mmol). A linear form of Equation (1) is as follows:
C L C s = 1 K L S m + C L S m
As such, the parameters Sm and KL can be determined via a linear regression. The fitted parameters are 167 and 251 mmol/kg in Sm and 0.36 and 72 L/mmol in KL for ARS sorption on Cal and HCal, respectively. The significant difference in KL values suggests that to achieve a low equilibrium ARS concentration, say 0.05 mM, the amounts of ARS removed would be 3 mmol/kg and 200 mmol/kg by Cal and HCal. This is about a 70 times increase in ARS removal. The Freundlich isotherm model has the following formula:
C S = K F C L 1 / n
It can be rearranged into a linear form:
L o g C S = 1 n l o g C L + l o g K F
where KF is the Freundlich isotherm constant and 1/n is the intensity of sorption. The KF values are 45 and 420, while the 1/n values are 0.9 and 0.3 for ARS removal by Cal and HCal, suggesting a favorable ARS removal by HCal. The sorption is favorable when 1/n is 0 < 1/n < 1 and unfavorable when 1/n is greater than 1 [35]. As such, ARS removal was favored by both Cal and HCal. For ARS sorption on bentonite, the Freundlich isotherm resulted in an r2 of 0.994 vs. r2 of 0.74 for the Langmuir sorption isotherm model [24].
The Temkin isotherm is expressed as
C S = R T B T l n ⁡ ( K T C L )
and it can be rearranged into a linear form:
C S = R T B T l n C L + R T B T l n K T
where R is the gas constant, T is the temperature in K, BT is the Temkin constant related to the heat of sorption (kJ/mol), and KT is the equilibrium binding constant (L/mol) [36]. The calculated BT and KT values are 278 and 60 kJ/mol and 3.5 and 7.1 L/mmol for ARS removal by Cal and HCal, respectively. All fitted parameters based on the three isotherm models are listed in Table 1. For comparison, the sorption of ARS on CaO, which was purchased from Fisher, is also shown in Figure 2, and the observed data were fitted to the Langmuir isotherm model, as it was speculated that CaO was produced after Cal was heated.
Most of the studies on ARS removal have focused on using modified or synthetic materials, such as polyethyleneimine-functionalized magnetic carbon nanotubes [37], as most Earth materials are negatively charged on their surfaces and ARS is an anionic dye. Thus, the net repulsive interaction prevented the use of raw Earth materials for ARS removal. Still, the sorptive removal of ARS was evaluated using LDH with an ARS removal of 29 mg/g [25] and using bentonite with a capacity of 170 mg/g [24]. In another study, an activated clay modified by iron oxide achieved an ARS removal capacity of 33 mg/g, but the XRD data showed the dominant mineral was quartz, and no clay mineral was detected [26]. The sorption of ARS on Fe2O3-coated biochar/kaolin reached a capacity of 175 mg/g [27]. In the current study, the ARS sorption capacity reached 251 mmol/kg or 86 mg/g on HCal based on the Langmuir isotherm fitting (Figure 2a), showing great promise for using Earth materials, or calcite in this case, for the removal of anionic contaminants after heat treatment.

3.2. Kinetics of ARS removal

Under different contact times, the amount of ARS removed is plotted in Figure 2b. Several kinetic models were utilized to fit the experimental data. The pseudo-second-order kinetic model has the formula
q t = kq e 2   t 1 + k q e t
which can be rearranged into a linear form:
t q t = 1 k q e 2 + 1 q e t
where k and kqe2 are the rate constant and initial rate of ARS removal by the solid with units of kg/mmol/h and mmol/kg/h, and qt and qe are the amounts of ARS removal at time t and at equilibrium with a unit of mmol/kg. For ARS removal by Cal from an initial concentration of 1.0 mM, the qe was 17 mmol/kg, while the k was 0.7 kg/mmol/h. After heat treatment, with an initial concentration of 6.0 mM, the qe increased to 238 mmol/kg, although the k was decreased by 50% to 0.35 kg/mmol/h. The kqe2 values are 213 and 20,000 mmol/kg/h for ARS removal by Cal and HCal (Table 1). For comparison, ARS removal by halloysite showed a qe value of 33.3 mmol/kg and a kqe2 value of 222 mmol/kg/h [5]. Again, the results show significant promise for using Earth materials for anionic contaminant removal after heat treatment.

3.3. Effect of Equilibrium Solution pH, Ionic Strength, and Temperature on ARS Removal

With pKa values of 5.49 and 10.85 [9], ARS could be in neutral form when the solution pH is less than its pKa1 value, monoanionic base when the solution pH is between its pKa1 and pKa2 values, and dianionic base when the solution pH is higher than its pKa2 values (Figure 1). However, the removal of ARS by Cal and HCal is less affected by equilibrium solution pH, varying from 220 to 240 mmol/kg for HCal and 7 to 9 mmol/kg for Cal (Figure 3a). A slight decrease in ARS removal (from 240 to 220 for HCal and 9 to 7 for Cal) was found at pH 12. Similarly, ARS sorption on an activated clay modified by iron oxide decreased slightly from 24.6 mg/g at pH 3 to 23.4 mg/g at pH 11 [26]. ARS sorption on a biosorbent of mustard husk decreased from 100% at pH 2 to 45–50% at pH 9–10 with initial concentrations of 25, 50, and 100 mg/L [38]. Higher pH values promoted the solubility of alizarin (AZ) in aqueous solution, as such, higher pH values negatively influenced AZ sorption on halloysite from 9.8 mg/g at pH 9 to 4.8 mg/g at pH 13 [39]. Similarly, ARS sorption on magnetite–talc composite materials decreased from 75% at pH 2 to 15% at pH 10 [23].
The solution ionic strength had a minimal influence on ARS removal by Cal and HCal. The ARS removal was about 240 mmol/kg for HCal in comparison to 9 mmol/kg for Cal, an increase of more than 25 times (Figure 3b). Still, the equilibrium ARS concentrations were less than 0.005 mM with an initial concentration of 6.0 mM when HCal was used, in comparison to equilibrium concentrations of 0.03 to 0.08 mM with an initial ARS concentration of 0.5 mM when Cal was used.
ARS sorption as affected by equilibrium temperature is depicted in Figure 3c. The thermodynamics for solute sorption from solution is governed by
ln K d = − Δ H R T + Δ S R
where Kd is the distribution coefficient of the solute, ΔH and ΔS are the changes in enthalpy and entropy after solute sorption on solid surfaces, and R and T are gas constant and temperature in K. The ΔG is the change in free energy after solute sorption and is related to ΔH and ΔS by
Δ G = Δ H − T Δ S
The ΔH values are 12 and 29 kJ/mol, the ΔS values are 0.06 and 0.16 kJ/mol/K, and the ΔG values are −10 to −13 and −20 to −25 kJ/mol for ARS sorption on Cal and HCal, respectively, suggesting a spontaneous and endothermic sorption, resulting in an increase in disorder of the sorbed ARS molecules on the surface, and the interactions between the ARS molecules and the surfaces of Cal and HCal are relatively weak, such as electrostatic interactions.

3.4. XRD Analyses

The XRD patterns of Cal and HCal after ARS sorption are presented in Figure 4. For reference purposes, the XRD pattern of CaO is also plotted in Figure 4. The literature showed some discrepancies in the locations and assignments of CaO diffraction peaks. In one study, the XRD peaks for CaO were located at 32.10°, 37.50°, and 54.20°, corresponding to (111), (200), and (311), with relative intensities of 30, 100, and 40 [40]. In a different study, the CaO peaks were listed at 18.02°, 34.12°, and 50.85° for the three highest intensities [41]. Another study showed peaks located at 32.5°, 36.6°, 38.2°, 44.5°, 45.4°, 64.2°, 67.8°, 77.4°, and 81.8° (JCPDS card No. 00-004-0777) [42]. The XRD peaks of CaO were at 29.59°, 32.22°, 37.35°, 42.81°, 53.83°, 64.09°, and 67.34°, corresponding to diffractions of (011), (111), (002), (012), (022), (113), and (222), agreeing with CaO (lime, syn, JCPDS card No. 00-004-0777) with a cubic crystal system and lattice parameters (5.8152 Å) [43]. In this study, they are located at 32.20°, 37.34°, and 53.86°, agreeing with those of Kumar et al. [40] (Figure 4). These peaks were not found in the HCal samples after being in contact with different ARS solutions for 24 h, suggesting that either CaO is not present in the system or is converted back into calcite after being in contact with ARS solutions for 24 h and then being air-dried.
From the XRD analyses of Cal and HCal, the following observation could be identified. Firstly, the d-spacing of Cal remained the same after it was heated to 1000 °C for 3 h. The (111) peak of CaO at 32.34° 2θ and the (101) peak of Ca(OH)2 at 34.11° 2θ [44] are not present in the XRD patterns of HCal either, suggesting that neither was produced after being heated at 1000 °C for 3 h or converted back into calcite after being shaken for 24 h in ARS solution. Meanwhile, an early study on the XRD patterns of ARS showed peaks at 26.38, 31.72, and 45.44° 2θ due to the reflections of (−304), (222), and (−434) [45]. Secondly, no diffraction peaks of CaO or ARS were found in these samples, suggesting that the mineral phase was not altered (remained as calcite) after heat treatment, and no crystalline ARS was found on Cal or HCal surfaces. Thirdly, the d-spacing of calcite did not change after ARS sorption, indicating that the sorption sites were on the external surfaces.

3.5. FTIR Analyses

For nano-CaO, the bands located at 3642 cm−1 are assigned to the O–H free hydroxyl bond from the residual hydroxide or the moisture content in the sample, the ones at 1471 cm−1 and 876 cm−1 indicate the C–O bond due to the carbonation of CaO nanoparticles, and the two small dips at 1229 cm−1 and 1633 cm−1 and the band at 2515 cm−1 are attributed to C–O stretch of the CO2 stretching [40]. In this study, these bands are very weak, suggesting that the CaO had low crystallinity.
For crystalline ARS, the bands are at 3448 cm−1 for ν(OH) due to hydrogen bonding, at 1634 cm−1 for ν (10-C=O), at 1589 cm−1 for ν (9-C=O), at 1546 cm−1 and 1499 cm−1 for ν (Ar C=C), at 1191 cm−1 for ν (C–O), at 1069 cm−1 for νas(SO3), at 1037 cm−1 for νs(SO3), and at 1127 cm−1 for carbonyl C–C–C [46]. None of these bands showed up on the FTIR spectra of Cal and HCal after ARS sorption (Figure 5). This could be due to the lower amounts of ARS on Cal or HCal surfaces.
For Cal, the sharp intense band at 873 cm−1 was assigned to the O–C stretching of CaCO3, and the band at 713 cm−1 is due to the OCO bending [47]. In this study, for the Cal samples, they were located at 872 and 712 cm−1 and stayed the same regardless of the amount of ARS sorption (Figure 5). However, for the HCal samples, they were located at 872 and 712 cm−1 when the ARS loading level was low and shifted to 874 and 727 cm−1 when the ARS loading was high (Figure 5). This may suggest slight changes in binding between ARS and HCal from low to high ARS sorption levels.

3.6. SEM Observation and ETF Scans

The SEM observation showed no changes in the particle morphology and particle size between Cal and HCal (Figure 6). In addition, the {104} cleavage is still visible and very well developed after Cal was heated to 1000 °C for 3 h (Figure 6b), confirming the XRD analyses. The element scan for S and Na was also conducted to see the sorption of ARS and the counterion Na+. A significant increase in the S density was noticed for HCal after ARS sorption (Figure 6d), in comparison to the S density on Cal after ARS sorption (Figure 6c). In contrast, the Na density was about the same for Cal and HCal after ARS sorption (Figure 6e,f), suggesting that the enhanced ARS sorption on HCal was not associated with counterion Na+ sorption.

3.7. TGA Analyses

The TGA analyses of the Cal and HCal can be seen in Figure 7. For Cal, the decomposition started at 710 °C and ended at 790 °C with a weight loss of 43%, agreeing well with standard calcite TGA data [48]. In contrast, the major decomposition for HCal started at 420 °C and ended at 460 °C. This could be due to the breakdown of Ca(OH)2. In a study of TGA analyses of CaO exposed to ambient moisture for several days, Ca(OH)2 decomposition between 350 and 400 °C was found [49]. However, in this study, it is almost identical to that of CaO (Figure 7a).
The second decomposition started at 600 °Cand ended at 630 °C. This could be due to the breakdown of calcite. Thus, the TGA results showed a significant difference between Cal and HCal. This could be caused by the fact that the HCal was air-dried in the oven and was never in contact with water, and the minute amount of calcite production could be due to the reaction of CaO with moisture and CO2 in the air during the storage process.
For Cal after sorption with ARS, the TGA results matched well with the raw Cal (Figure 7b). However, for the HCal after equilibration with different initial concentrations of ARS, the TGA data showed different patterns to those of HCal, i.e., the decomposition temperature corresponding to CaO disappeared (Figure 7c,d), but were almost identical to the TGA of Cal, suggesting that after being in contact with water or ARS solution for 24 h and then being dried in the air, there is a significant conversion back from CaO to calcite. It was previously reported that after Cal was heated to 900 °C, CaO was produced, and in the presence of moisture, the CaO would convert to Ca(OH)2 [50]. This study suggested that CaO produced after Cal decomposition at 900 °C could be converted back into calcite.

4. Discussion

The interaction of Ca2+ with ARS was used for Ca stain [16]. Thus, due to the presence of Ca2+ on the Cal surfaces, it was anticipated that Cal would be able to sorb a significant amount of ARS. However, the preliminary study using 0.1 and 0.2 mM ARS for its sorption on Cal and HCal heated to 600 and 800 °C resulted in similar sorption (about 70%) removal. To achieve higher ARS removal, Cal was heated to 1000 °C in this study. The ARS removal capacities are 167 and 251 mmol/kg, but the affinity values of ARS for Cal and HCal are 0.36 and 72 L/mmol. The significant difference in KL values suggests that HCal is extremely effective in the removal of ARS at low initial concentrations. To achieve an equilibrium ARS concentration of 0.05 mM, the amounts of ARS removed would be 3 mmol/kg or 200 mmol/kg when Cal or HCal were used.
The solution pH played an important role in ARS sorption onto the {104} cleavage of Cal. At a low pH, only one hydroxyl group is deprotonated, resulting in a possible adsorption configuration mediated by the 3-C–O− and 4-C–OH groups [51]. When the pH is above 8, both hydroxyl groups are deprotonated, and the ARS molecules bind to two C–O− groups on the surface of Cal. However, the solution pH had a minimal effect on ARS removal in this study, suggesting that the {104} cleavage may not play an important role as the ARS sorption surface.
A previous study showed that Ca can react with ARS via its sulfonic acid and/or its OH groups [16]. Under different solution pH, the relative amount of the three ARS species would be different. With an increase in the pH, electrochemical reduction may lead to the dissociation of four phenolic hydroxyl groups, resulting in the formation of seven different species, with negative charges located on different O [52]. In addition, the binding between Ca2+ and ARS could be of salt type, by which the Ca2+ complexes with O atoms attached to C1 and C2, or of chelate type, by which the Ca2+ complexes with O atoms attached to C1 and C9 [9]. For the former case, the binding between Ca2+ and ARS could be 1:1, or 1:2 in a dimeric formation (Figure 8). These were just speculations based on the amount of ARS sorbed on Cal and HCal.
As the reaction between ARS and Ca2+ was widely used for Ca stain [35], it was anticipated that calcite would be able to sorb ARS due to the presence of Ca2+ on the surface. However, the ARS sorption on Cal was much lower in comparison to HCal. In a previous study, when ARS sorbed on HDol surfaces, it was speculated that salt-type or chelate-type interactions between Ca2+ and ARS were responsible for the enhanced ARS removal by HDol [53]. In this study, the same type of interactions may also contribute to the much-enhanced ARS removal by HCal. On the other hand, as the XRD pattern of HCal before being added to water showed dominant CaO peaks, the significant increase in ARS removal by HCal could be due to the interactions between ARS and Ca2+ from CaO, which could be converted back to calcite in aqueous media. This mechanism would also explain why the mineral phase was still calcite for HCal samples after ARS sorption (Figure 4 and Figure 5). Previous results showed that the ARS sorption capacity on CaO was up to 1 mg/g [54]. In this study, the ARS sorption capacity was 251 mmol/kg, or 86 mg/g. This is significantly higher than in the previous study [54]. As such, the heat treatment of calcite not only increased the ARS removal in comparison to Cal but also in comparison to CaO, which is reported for the first time in this study.

5. Conclusions

In this study, the removal of color dye ARS by calcite after heat treatment was evaluated in detail under different physico-chemical conditions. The ARS removal capacity increased from 167 mmol/kg to 251 mmol/kg, a 50% increase, following the heat treatment of calcite. Additionally, the Langmuir coefficient increased from 0.36 to 72 L/mmol, an increase of 200 times. Such an increase would make HCal much more effective in removing ARS. The removal of ARS by HCal could be attributed to the formation of complexation with Ca2+ on the solid surface. Thus, the heat treatment of calcite in this study indicated a new approach and the effective utilization of Earth materials such as limestone for the removal of industrial contaminants from water in addition to their general use as raw materials for manufacturing cement and as construction materials.

Author Contributions

Conceptualization, Z.L. and S.X.; methodology, L.A., Z.L. and S.X.; investigation, A.R., A.B. and J.S.; resources, Z.L.; data curation, A.R., A.B. and J.S.; writing—original draft preparation, Z.L.; writing—review and editing, Z.L.; visualization, Z.L.; supervision, Z.L.; project administration, Z.L.; funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by (1) the Sparks grant from WiSys, for dye removal using carbonated minerals; (2) a grant from WiSys for Scientist in Residency; and (3) a grant from Fresh Water Collaborative of Wisconsin to Engaging Undergraduate Students in Research on using Earth Materials for contaminant removal including dyes and per- and polyfluorinated substances.

Data Availability Statement

Due to patent application, the data pertaining to the original contributions presented in this study are stored at the University of Wisconsin–Parkside. Further inquiries can be directed to the corresponding author.

Acknowledgments

The assistance from Justin J. Peschman with SEM imaging is acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Molecular structure of ARS (a) and its pH–speciation diagrams (b).
Figure 1. Molecular structure of ARS (a) and its pH–speciation diagrams (b).
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Figure 2. Sorption isotherms (a) and kinetics (b) of ARS on raw Cal (red circle) and HCal (blue diamond). The Langmuir, Freundlich, and Temkin fitting are indicated by black, red, and blue lines in (a). The ARS sorption on CaO is fitted to the Langmuir isotherm only (purple line).
Figure 2. Sorption isotherms (a) and kinetics (b) of ARS on raw Cal (red circle) and HCal (blue diamond). The Langmuir, Freundlich, and Temkin fitting are indicated by black, red, and blue lines in (a). The ARS sorption on CaO is fitted to the Langmuir isotherm only (purple line).
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Figure 3. Influence of equilibrium solution pH (a), ionic strength (b), and temperature (c) on ARS sorption on Cal (red circle) and HCal (blue diamond).
Figure 3. Influence of equilibrium solution pH (a), ionic strength (b), and temperature (c) on ARS sorption on Cal (red circle) and HCal (blue diamond).
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Figure 4. XRD patterns of Cal (a) and HCal (b) after ARS sorption at different concentrations.
Figure 4. XRD patterns of Cal (a) and HCal (b) after ARS sorption at different concentrations.
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Figure 5. FTIR spectra of Cal (a) and HCal (b) after ARS sorption at different initial concentrations. The transmittance is in arbitrary value units due to overlay of the FTIR spectra vertically.
Figure 5. FTIR spectra of Cal (a) and HCal (b) after ARS sorption at different initial concentrations. The transmittance is in arbitrary value units due to overlay of the FTIR spectra vertically.
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Figure 6. SEM images of Cal (a) and HCal (b), the ETF scan of S on Cal (c) and HCal (d), and the ETF scan of Na on Cal (e) and HCal (f).
Figure 6. SEM images of Cal (a) and HCal (b), the ETF scan of S on Cal (c) and HCal (d), and the ETF scan of Na on Cal (e) and HCal (f).
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Figure 7. TGA and DTG analyses of Cal, HCal, and CaO (a), Cal (b) and HCal (c,d) at different initial concentrations. The left y-axis is for TGA, and the right y-axis is for DTG. The blue lines in (b–d) are a 20% upshift of the left y-axis and a 0.2%/°C upshift of the right y-axis.
Figure 7. TGA and DTG analyses of Cal, HCal, and CaO (a), Cal (b) and HCal (c,d) at different initial concentrations. The left y-axis is for TGA, and the right y-axis is for DTG. The blue lines in (b–d) are a 20% upshift of the left y-axis and a 0.2%/°C upshift of the right y-axis.
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Figure 8. Illustration of electrostatic interaction (left) or complexation interaction (right) of ARS on Cal or HCal surfaces.
Figure 8. Illustration of electrostatic interaction (left) or complexation interaction (right) of ARS on Cal or HCal surfaces.
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Table 1. Parameters of ARS uptake on Cal and HCal from isotherm and kinetic studies.
Table 1. Parameters of ARS uptake on Cal and HCal from isotherm and kinetic studies.
Sorption ParametersARS Sorption on CalARS Sorption on HCal
Sm (mmol/kg)167251
KL (L/mmol)0.3672
r2 for Langmuir isotherm fitting0.460.92
KF (L/kg)45420
1/n 0.90.3
r2 for Freundlich isotherm fitting0.990.91
BT (kJ/mol)27860
KT (L/mmol)3.57.1
r2 for Temkin isotherm fitting0.840.87
qe (mmol/kg)17238
kqe2 (mmol/kg-h)21320,000
k (kg/mmol-h)0.70.35
r2 for pseudo-second-order fitting0.99990.9999
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Li, Z.; Bowman, A.; Rayniak, A.; Strommen, J.; Allen, L.; Xu, S. Heat Treatment of Calcite to Enhance Its Removal of Color Dye Alizarin Red S. Crystals 2024, 14, 450. https://doi.org/10.3390/cryst14050450

AMA Style

Li Z, Bowman A, Rayniak A, Strommen J, Allen L, Xu S. Heat Treatment of Calcite to Enhance Its Removal of Color Dye Alizarin Red S. Crystals. 2024; 14(5):450. https://doi.org/10.3390/cryst14050450

Chicago/Turabian Style

Li, Zhaohui, Anna Bowman, Angie Rayniak, Jadyn Strommen, Lori Allen, and Shangping Xu. 2024. "Heat Treatment of Calcite to Enhance Its Removal of Color Dye Alizarin Red S" Crystals 14, no. 5: 450. https://doi.org/10.3390/cryst14050450

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